The objectives of this research project are to understand the folding, mechanism, and evolutionary relationships between redox metalloproteins such as cytochromes, and to investigate the evolutionary development of the respiratory and photosynthetic electron transport chains of which they are a part. To this end we have solved the crystal structures of reduced cytochrome c from eukaryotes to 2.0 angstrom units resolution and the oxidized form to 2.8 angstrom units, and are in the process of taking the oxidized cytochrome structure down to 2.0 angstrom units. From this work a model has been proposed for the reduction and oxidation of the protein in its respiratory chain. We are also interested in tracing the molecular folding found in eukaryotic cytochrome c back to the photosynthetic and respiratory machinery of prokaryotes. We have solved the structure of oxidized cytochrome c550 from Micrococcus denitrificans, and have determined its amino acid sequence by standard chemical techniques. The x-ray analysis of a smaller c551 from Pseudomonas aeruginosa is well under way (its sequence is known). We hope to extend these studies to cytochromes of sulfate respiration in bacteria, and possibly to components of bacterial photosynthesis. A "West Coast redox group" is being organized around the x-ray laboratories at Caltech, UC San Diego, Arizona, and Washington, to coordinate the study of cytochromes and other components of electron transport chains in bacteria and higher organisms.